PROSPECTUS SUPPLEMENT
(To Prospectus Dated January 28, 1998)

                         807,746 Shares of Common Stock
                                ($.50 Par Value)

                               OMNICOM GROUP INC.

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      This document  supplements the Prospectus  dated January 28, 1998 relating
to 807,746 shares of Common Stock, par value $.50 per share (the "Common Stock")
of the Omnicom Group Inc., a New York corporation  ("Omnicom" or the "Company").
Each  share of Common  Stock  offered  hereby  was issued  upon  exchange  of an
Exchangeable Share (an "Exchangeable Share") of GPC International  Holdings Inc.
("GPC"),  a  corporation  incorporated  under  the laws of  Canada,  issued in a
private  offering in Canada by GPC and the  Company.  The shares of Common Stock
offered hereby were initially acquired upon exchange of the Exchangeable  Shares
in a private offering in Canada.  This Prospectus  Supplement is incorporated by
reference into the Prospectus,  and all terms used herein shall have the meaning
assigned to them in the  Prospectus.  On July 21, 1998 the closing  price of the
Common  Stock as reported on the New York Stock  Exchange  was $54.25 per share.
The Common Stock is traded under the symbol "OMC."

      The Selling  Shareholder  does not own in excess of 1% of the Common Stock
of the Company and since the Selling  Shareholder  may sell all, some or none of
the  shares of Common  Stock  offered  hereby,  no  estimate  can be made of the
aggregate  number of shares of Common  Stock  that will be owned by the  Selling
Shareholder upon completion of the offering to which this Prospectus  Supplement
relates.  In accordance  with the Section of the  Prospectus  entitled  "Selling
Shareholders"  (which  appears  on page  10 of the  Prospectus),  the  following
information  is provided  with  respect to the  beneficial  owners of the Common
Stock:

                                          Amount of Shares          Amount of
                                         Beneficially Owned       Shares to be
Name of Selling Shareholder(s)         as of July 21, 1998(1)   Offered for Sale
- ------------------------------          --------------------    ----------------
1067913 Ontario Limited (2)                    52,560                  17,500

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(1)   Includes  35,060 shares of Common Stock which 1067913  Ontario Limited has
      the right to acquire upon the exchange of 35,060 Exchangeable Shares.

(2)   James  Crossland,  the  controlling  shareholder,  is  an  officer  and/or
      director of GPC and a subsidiary of GPC and indirectly  owns 290 shares of
      Common Stock.

      Except for the purchase of the shares of Common Stock and the  information
disclosed in the footnotes above, the Selling Shareholder has not had a material
relationship  with the  Company or any of its  affiliates  within the past three
years.

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      THESE  SECURITIES  HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES
AND EXCHANGE  COMMISSION OR ANY STATE SECURITY COMMISSION NOR HAS THE SECURITIES
AND  EXCHANGE  COMMISSION  OR ANY STATE  SECURITIES  COMMISSION  PASSED UPON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.

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            The date of this Prospectus Supplement is July 23, 1998.